1. Field of the Invention
The present invention relates to a continuous-sheet printing tandem electrophotography system having a plurality of electrophotography apparatuses coupled to one another for printing a continuous sheet of a recording medium, and a method of printing a continuous sheet. In particular, the present invention relates to the correction of a print position error between the both sides of a printed continuous sheet.
2. Description of the Related Art
FIG. 1 schematically illustrates the principle of operation of a conventional electrophotography apparatus. A laser light source 301 is turned on or off in accordance with image data transmitted in synchronization with a video clock. The laser light source 301 emits a laser beam 302 that is reflected by a polygon mirror 303 as it rotates at a certain angular velocity, thereby scanning the surface of a photosensitive drum 304 rotating at a predetermined velocity with the laser beam 302. As a result, a latent image is formed on the surface of the photosensitive drum 304.
A beam detector 305 is disposed along the scanning line of the laser beam 302. Upon detection of the laser beam 302, the beam detector 305 outputs a horizontal synchronization signal, in accordance with which the output timing of the image data is determined so that an accurate write start position can be obtained.
The latent image on the photosensitive drum 304 is then developed using a magnetic brush of a two-component developer consisting of a mixture of a toner 306 and a carrier at a certain ratio. Specifically, the toner 306 is caused to attach to the surface of the photosensitive drum 304, thereby making the latent image visible as a toner image.
A continuous sheet 308 is transported by tractors or rollers 307 at a speed corresponding to the circumferential speed of the photosensitive drum 304, and the toner image on the photosensitive drum 304 is transferred onto the continuous sheet 308 by a transfer unit 309. The toner image on the continuous sheet 308 is then fused thereon by pressing and heating by a fusing unit including rollers 310, thus completing the print process.
In this case, it is necessary to synchronize the rotating speed of the polygon mirror 303 as it reflects the laser beam, the rotating speed of the photosensitive drum 304, and the sheet transport speed. For this purpose, a single oscillator is generally used. Specifically, the individual devices are driven in accordance with a control clock, so that their relative synchronization can be ensured as long as the control clock is generated by the same oscillator. If the devices are controlled by different oscillators, the difference in the clock signals accumulates in the continuous-sheet electrophotography apparatus and the devices lose synchronization, rendering the realization of normal apparatus performance impossible.
The frequency of the control clock is uniquely determined by the optical specifications of the apparatus, a sheet transport speed which is equivalent to the print speed, and the photosensitive drum rotation speed. Another condition is that there should be only one oscillator, as mentioned above. Thus, the oscillating frequency is calculated from the least common multiple of the clock frequencies required by the individual devices, and an appropriate crystal oscillator is selected from the viewpoint of accuracy.
A continuous-sheet printing tandem electrophotography system is known in which a couple of continuous-sheet electrophotography apparatuses of the aforementioned type are disposed upstream and downstream along the transport of a continuous sheet, for printing both sides of the sheet, for example. Such a system has a market under the category of electrophotography equipment as a relatively simple commercial printing machine capable of high-speed, high-availability, and low-cost operations. Although there are also special-purpose offset printing machines, such as rotary presses, these are designed to compensate for the time-consuming setup process with the number of printed pages and are therefore not suitable for low-volume production. Thus, a small-volume, small-lot commercial printer market is being developed in which electrophotography systems and offset printing machines are competing against each other.
There has recently been a growing demand for coupling a plurality of continuous sheet electrophotography apparatuses for printing. FIG. 2 schematically shows a continuous-sheet printing tandem electrophotography system. In this system, two continuous-sheet electrophotography apparatuses of the type shown in FIG. 1 may be coupled and used in various combinations. For example, an upstream device 401 to the right in FIG. 2 prints an upper surface of a continuous sheet, followed by the printing of a lower surface by a downstream device 402 to the left, thus forming a double-side printing system. Alternatively, the upstream device 401 may use black toner while the downstream device 402 may use a color toner, thereby forming a spot color printing system. In the illustrated example, a sheet inverting unit 403 is provided between the upstream device 401 and the downstream device 402, forming a double-side printing system.
One drawback of this system is that when a double-side printing is performed, thermal contraction of the sheet occurs in the fusing unit of the upstream device 401, so that a print position error is caused when the lower surface is printed by the downstream device 402. Solution of the problem is earnestly desired because the above system enables the small-volume, small-lot production of printed matter for commercial printing purposes by a simple operation.
Various methods for correcting the contraction of the sheet have been proposed, such as Japanese Laid-Open Patent Application Nos. 2004-347842 and 2005-186614 teaching controlling the operating frequency of a laser clock, the speed of a polygon mirror motor, or the PWM output of laser power. However, these methods are all directed to electrophotography apparatuses using cut-sheets, where the upper and lower surfaces of a cut-sheet are printed in a single printing system along separate time axes by switching control values and by inverting the cut-sheet. Although the time for transition between the control values is ensured during the time of no printing between pages, the conventional methods do not take into consideration the decrease in throughput, which is a serious concern from the viewpoint of commercial printing. Further, the aforementioned related art does not provide any quantitative definition concerning main and sub scan operations and laser power correction.
In a continuous-sheet tandem printing system using a continuous sheet, the operation of one printing unit may need to be temporarily stopped when the individual printing units are allocated different numbers of pages to process, thus resulting in a decrease in throughput. If a sheet stays between the upper- and lower-surface print units, problems other than a print quality problem may be caused. Therefore, it is necessary for the upper- and lower-surface print units to process the same number of pages along the same time axis, and to achieve print position alignment between the lower and upper surfaces when a sheet contraction develops.